The Milky Way's four galactic quadrants as depicted in Star Trek along with labels noting locations of the major political forces

In the television series Star Trek and its spin-offs, a galactic quadrant refers to an area of the Milky Way Galaxy. In the original Star Trek, it referred to an area interchangeable with a sector. However, in the various spin-off series and movies beginning with Star Trek: The Next Generation, it refers to a system of four galactic quadrants, designated by the Greek letters Alpha, Beta, Gamma, and Delta.

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The original usage of quadrant occurs in episodes such as "The Deadly Years" and "The Squire of Gothos", where numbered quadrants (448 and 904) are given.[1] In other episodes it appears to refer to an entire quarter of the galaxy.[2] The season 3 Star Trek: The Next Generation Writers' Technical Manual asserts that there are four quadrants in each sector (which seem to vary in size).[3]

The exact quadrant borders are not clearly established in the series canon. Episodes often contradict each other. Art staff explanations also contradict each other. The 1988 Star Trek: The Next Generation episode "The Price" introduced the concept of four quadrants of the galaxy, named the Alpha, Beta, Gamma and Delta quadrants.

Assuming the galactic meridian runs from the center of the galaxy through the Solar System, then the Alpha Quadrant is between 180 degrees and 270 degrees, and the Beta Quadrant is between 90 degrees and 180 degrees.

Star Trek Star Charts has the Romulans corewards and counterclockwise from the Federation core, with the Klingons rimwards of the Romulans. Federation territory is shown to expand around the backs of the Klingon and Romulan Empires, further into the Beta Quadrant. Other powers such as the Cardassians, Tholians and Ferengi are on the clockwise side of the Federation. The 1975 Star Fleet Technical Manual by Franz Joseph also has a galactic meridian running through Sol, but depicted the Romulans as being counterclockwise, and the Klingons as being clockwise.

Although the Romulans and Klingons are shown by maps as part of the Beta Quadrant, Star Trek: Deep Space Nine did not use this terminology, referring to them as Alpha Quadrant powers. Ronald D. Moore noted that the "Alpha quadrant is firmly planted in the audience's mind as where the Federation, the Klingons, and the Romulans all live so I think we'll stick with that nomenclature".[5]

About 600,000 years ago, the Alpha Quadrant was dominated by the Tkon Empire, which is long since vanished by the time of the Star Trek series.

In the fictional Star Trek universe, the Bajoran wormhole is a unique spatial anomaly located near the planet Bajor. The Bajoran religion refers to the wormhole as the "Celestial Temple". It appears as an aperture of swirling golden light surrounded by blue clouds, which appears when a vessel approaches and disappears when the vessel enters it; it also appears when discharging a vessel. The wormhole can only be traversed by ships traveling at impulse (sub-light speed) velocities.

The wormhole is discovered in the first episode of Star Trek: Deep Space Nine. It is found to lead from the Bajor-B'Hava'el system in the Alpha Quadrant to the Idran system in the Gamma Quadrant, 70,000 light-years away and on the other side of the galaxy. Due to the strategic importance of such a phenomenon, the Deep Space Ninespace station is moved out of Bajor's orbit and repositioned near the wormhole.

Starfleet Commander Benjamin Sisko and Lt. Jadzia Dax are the first to make contact with the wormhole and its creators, noncorporeal beings known as the Prophets in Bajoran religion; they are simply referred to as "wormhole aliens" by the Federation. It is they who provided the Orb-like energy artifacts (or "Tears of the Prophets") to the Bajorans and these artifacts are the basis of Bajoran religion.

The wormhole becomes a point of conflict throughout the series, since it permits travel between the Alpha Quadrant and the Gamma Quadrant controlled by the Dominion, a militaristic interstellar state led by the race of shapeshifters known as the Changelings. Neither the Dominion nor the major powers of the Alpha Quadrant wish the other to gain a foothold in their relative quadrants, which explains the strategic importance of the wormhole running between them. The wormhole closes at one point, as a result of one of the Tears of the Prophets being temporarily rendered inert by the Pah Wraiths. It is later restored by the discovery of an additional orb by Benjamin Sisko.

Most of the information about the Delta Quadrant and its inhabitants comes from the show Star Trek: Voyager. Apart from the voyages of the USS Voyager, the USS Equinox, the USS Raven, briefly the USS Enterprise-D, and the Federation citizens assimilated or de-assimilated from the Borg, the Federation has not explored this quadrant.